Butanol is an important industrial chemical, useful as a fuel additive, as a feedstock chemical in the plastics industry, and as a food-grade extractant in the food and flavor industry. Each year 10 to 12 billion pounds of butanol are produced by petrochemical means and the need for this commodity chemical will likely increase. 2-Butanone, also referred to as methyl ethyl ketone (MEK), is a widely used solvent and is the most important commercially produced ketone, after acetone. It is used as a solvent for paints, resins, and adhesives, as well as a selective extractant and activator of oxidative reactions.
Methods for the chemical synthesis of 2-butanone are known, such as by dehydrogenation of 2-butanol or in a process where liquid butane is catalytically oxidized giving 2-butanone and acetic acid (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCHVerlag GmbH and Co., Weinheim, Germany, Vol. 5, pp.727-732). 2-Butanone may also be converted chemically to 2-butanol by hydrogenation (Breen et al., J. or Catalysis 236: 270-281 (2005)). Methods for the chemical synthesis of 2-butanol are known, such as n-butene hydration (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCHVerlag GmbH and Co., Weinheim, Germany, Vol. 5, pp. 716-719). These processes use starting materials derived from petrochemicals and are generally expensive, and are not environmentally friendly. The production of 2-butanone and 2-butanol from plant-derived raw materials would minimize greenhouse gas emissions and would represent an advance in the art.
Methods for producing 2-butanol by biotransformation of other organic chemicals are also known. For example, Stampfer et al. (WO 03/078615) describe the production of secondary alcohols, such as 2-butanol, by the reduction of ketones which is catalyzed by an alcohol dehydrogenase enzyme obtained from Rhodococcus ruber. Similarly, Kojima et al. (EP 0645453) describe a method for preparing secondary alcohols, such as 2-butanol, by reduction of ketones which is catalyzed by a secondary alcohol dehydrogenase enzyme obtained from Candida parapsilosis. Additionally, Kuehnle et al. (EP 1149918) describe a process that produces both 1-butanol and 2-butanol by the oxidation of hydrocarbons by various strains of Rhodococcus ruber. The process favored 1-butanol production with a selectivity of 93.8%.
The production of 2-butanol by certain strains of Lactobacilli is also known (Speranza et. al. J. Agric. Food Chem. (1997) 45:3476-3480). The 2-butanol is produced by the transformation of meso-2,3-butanediol. The production of 2-butanol from acetolactate and acetoin by these Lactobacilli strains was also demonstrated. However, there have been no reports of a recombinant microorganism designed to produce 2-butanol.
There is a need, therefore, for environmentally responsible, cost-effective processes for the production of 2-butanol and 2-butanone. The present invention addresses this need through the discovery of recombinant microbial production hosts expressing 2-butanol and 2-butanone biosynthetic pathways.